1. Field of the Invention
The invention relates in general to timing recovery of a receiver, and more particularly to a feedback convergence mechanism for timing recovery.
2. Description of the Related Art
In modern communication technologies, both a transmitting end and a receiving end employ a communication protocol or standard understood by both parties to promote the communications between the two parties. A signal transmitted by the transmitting end passes through a transmission channel and is received by the receiving end. In many communication protocol standards, a message to be transmitted is transmitted in form of chucks. In different communication protocols, these chucks may be referred to as packets, symbols, or other terms. In the disclosure, for better illustrations, these data chucks are referred to as symbols.
A communication protocol specifies a transmission speed or timing of these symbols. In other words, the transmitting end knows the speed according to which the symbols are to be transmitted, and the receiving end is also aware of receiving the symbols at the same speed. However, due to various realistic factors, the receiving end may not be able to synchronize the reception timing to be consistent with the transmitting end.
For example, a communication protocol standard may specify that transmission is to be performed at a speed of 1000 symbols per second. However, a clock oscillator at the transmitting end may not generate a timing of exactly 1 KHz. In practice, a timing error inevitably exists in the clock oscillator. In one ambient temperature, the clock oscillator at the transmitting end may exactly generate a perfect timing specified by the communication protocol. Yet, due to heat energy generated by constant operations of the transmitting end or a change in the ambient temperature, a change in the timing generated by the clock oscillator is unavoidably caused.
Similarly, the receiving end also requires a clock oscillator to generate the timing specified by the communication protocol. Same as the issue that the transmitting end encounters, the clock oscillator at the receiving end may not perfectly generate the timing specified by the communication protocol. In other words, although the timing specified by the communication protocol is 1 KHz, assuming that the transmitting end transmits the symbols at a timing of 1.001 KHz, the receiving end is also required to receive the symbols at a timing of 1.001 KHz. Assuming that the transmitting end transmits the symbols at a timing of 0.999 KHz, the receiving end is also required to receive the symbols at a timing of 0.999 KHz. Assuming that the receiving end is limited to operate at a timing of 1 KHz, complications may arise in the reception operations of the symbols.
FIG. 1 shows a schematic diagram of a signal propagation model in the prior art. Signals are transmitted by a transmitting end 110. These signals include multiple symbols, each of which being represented by Ik, where the subscript k represents a serial number. A pulse shaping function P(x) outputs the symbols in form of pulses, and a transmission time length required by each symbol is Tsym, tx. A signal sequence transmitted by the transmitting end 110 is denoted as x(t).
The signal x(t) is transmitted via a channel 120 to a receiving end 130. In real situations, the channel 120 is imperfect as it receives distortion of a multipath effect h(t) and random interferences. The latter is usually referred to as an additive Gaussian white noise (AWGN), which is denoted as w(t).
Having passed through the distorted and interfered channel 120, a signal received by the receiving end 130 is denoted as y(t). The signal y(t) is sampled by a sampling rate 1/Tsam to obtain a sampled signal y(n). The sampled signal y(n) is forwarded by the receiver 130 to a timing recovery module 132. An effect of the timing recovery module 132 is to synchronize the timing to the frequency for transmitting the symbols by the transmitting end 110, such that y(n)=y(t)|t=n:Isym,rx. The signal y(n) having passed through the timing recovery module 132 is forwarded to a subsequent processing unit, e.g., an equalizer 134, to decode and obtain a symbol . In an ideal situation, the symbol  is equal to the symbol Ik transmitted from the transmitting end 110.
In general, the above sampling rate is usually faster than the frequency at which the symbols are transmitted. With the timing recovery module 132, the frequency is down-converted to the so-called baseband. Therefore, a process for processing the signal y(n) having passed through the timing recovery module 132 by a subsequent processing unit is referred to as baseband processing.
The above details describe an ideal signal propagation model. As previously stated, the clocks generated by the clock oscillators of the transmitting end 110 and the receiving end 130 are not necessarily the same. An event of same clocks generated by the clock oscillators of the transmitting end 110 and the receiving end 130 may be purely regarded as a coincidence. In other words, in the signals sent from the clock recovery module 132, the time Tsym, rx occupied by each symbol received by the receiving end does not perfectly equal to the time Tsym, tx occupied by each symbol transmitted from the transmitting end. After a period of time, start time boundaries of the symbols may fail to align and synchronize, such that the synchronization of the symbols may become discrete and thus bring problems in the communication.
Therefore, to synchronize the timings of the transmitter end 110 and the receiving end 130, there is a need for a feedback mechanism for the timing recovery module 132 to allow the receiving end 130 to more precisely synchronize with the timing of the transmitting end 110, i.e., to have Tsym, tx to approximate Tsym, tx.